1. Field of the Invention
The present invention relates to a rate earth borate phosphor and a process for the preparation thereof, and the present invention also relates to a fluorescent screen comprising this phosphor.
2. Description of the Prior Art
As the known rare earth borate phosphor, there can be mentioned, for example, a phosphor disclosed in Japanese Patent Application Laid-Open Specification No. 33986/78, which is represented by the following general formula: EQU La.sub.1-x-y-z Gd.sub.x A'.sub.y A".sub.z B.sub.3 O.sub.6
wherein A' is Bi or Ce, A" is Tb or dy, x is a number of 0.ltoreq.x.ltoreq.1, y is a number of 0.001.ltoreq.y.ltoreq.0.80 and z is a number of 0.ltoreq.z.ltoreq.0.60, with the proviso that x, y and z satisfy the requirement of x+y+z.ltoreq.1.
The host lattice of this phosphor is composed of a lanthanoid sesquioxide Ln.sub.2 O.sub.3 (in which Ln is La or Gd) and B.sub.2 O.sub.3 at a molar ratio of 1:3 and the phosphor emits a peculiar fluorescence determined by the activator A' and/or A". As another rare earth borate phosphor, there can be mentioned Gd.sub.2 O.sub.3.B.sub.2 O.sub.3, La.sub.2 O.sub.3.B.sub.2 O.sub.3 activated with trivalent europium (Eu.sup.3+), which is disclosed J. Electrochem. Soc., 111, 12, page 1363 (1964). The host lattice of the phosphor of this type is composed of a lanthanoid sesquioxide Ln.sub.2 O.sub.3 (in which Ln is Gd or La) and B.sub.2 O.sub.3 at a molar ratio of 1:1 and the phosphor emits a red fluorescence inherent of Eu.sup.3+.
These known phosphors, however, are still insufficient in the brightness and the maintenance of fluorescent characteristics.